The wing rotors of wind turbines may be equipped with only a single wing blade having a corresponding counter balancing mass. Preferably, however, two wing blades are provided. But also rotors having three or more wing blades may be used.
Unless special measures are taken to ensure their smooth running, wind turbines run erratically due to different air flow speeds impinging on the turbine rotor in the region of the rotor plane of revolution. These speed differences predominantly result from the fact that wind velocity is inhibited near the ground friction boundary layer and it increases above the ground. In this case, the statistical mean values of air flow velocity in the upper region of the rotor plane of revolution can exceed those values in the lower region by such an amount that the aerodynamic forces against a wing blade during one rotor revolution can fluctuate upwardly or downwardly by more than 20%. Such cyclic force fluctuations are particularly dangerous because the cycle speed may be at the characteristic resonant frequencies of individual components, e.g., at the characteristic resonant frequencies of the wing blades or of the tower on which the wind turbine is permanently based. Under certain conditions, therefore, the durability limit of the components under load can be reached. Additionally, under gusty wind conditions, overloads can occur. These, too, can adversely affect single wing blades. Frequently, a sudden increase in wind velocity puts a load on the rotor during only part of its revolution.
In a known wind turbine shown in U.S. Pat. No. 2,360,791, an attempt has been made to damp the oscillations produced by the above phenomena. Each wing blade is individually coupled with the rotor hub by a respective link. The axis of each link is placed perpendicular to the rotational axis of the hub. A shock absorber equipped lever system allows for limited swiveling motion of the wing blades. A severe drawback of this design is that both mass unbalanced and aerodynamic unbalanced effects are produced by an unequal amount of individual wing blade deflections.
In another known wind turbine (described in Lueger: Lexikon der Technik (Engineering Encyclopedia), 1965, Vol. 7, pp. 574-581), the wing blades are rotatable around their longitudinal axes, in contrast to the above described design, but are otherwise rigidly supported in the rotor hub. To improve their smooth running in this case, a pivotable connection is provided between the rotor hub and the rotor shaft. The articulated axle of the pivotable connection is arranged perpendicular to the plane determined by both wing axes, namely so that the articulated axle intersects the rotor rotational axis at the center of gravity of the wing rotor (the latter consisting of the rotor hub and both wing blades). With this design, the wing rotor always swivels as a whole around its center of gravity, so that the mass unbalance is eliminated. In other respects, this known design operates as follows. If different aerodynamic forces engage the wing blades, then the pitching moment that is the resultant of the differential between the axial components of the aerodynamic forces triggers a swiveling motion of the rotor around its articulated axle. But, this pitching moment is constantly compensated for by the centrifugal forces constantly acting on both wings, as a function of the rotor rpm. The centrifugal forces produce a restoring moment that counteracts the above described pitching moment following each swiveling drift of the rotor from its standard position.
For rotor rpm control, there is a wing timing device, which preferably has a centrally placed setting rod connected to the wing blades via two control levers. Upon swiveling of the rotor around its articulated axle, the setting rod retains its position, so that the wing blades are timed, meaning in this context that the angle of attack of the higher wind flow velocity affected wing blade is relatively reduced while the angle of attack of the other wing blade is correspondingly increased. In this case, the coupling of both rotational motions, namely the swiveling motion of the wing rotor around its articulated axle and the rotational motion of each wing blade around its own longitudinal axis, is a motion that is mechanically identical to a swiveling motion of both wing blades around an axis. The latter axis is inclined at an angle to the above noted articulated axle which angle is determined by the rotating vector resultant from the swivel motion of the rotor and the turning of the wing blade. Through this measure, namely the coupling between swivel motion and wing blade rotation, a matching of wing blade engaging differential-tangential aerodynamic forces could be accomplished, but only under certain fixed operating conditions. With large-scale installations, there is an added complication that the disturbing force resultant from a residual differential between tangential aerodynamic forces is so large and the specific stiffnesses of loaded components are so negligible because of their large dimensions that there is still the danger of triggering coupled wing blade and/or tower oscillations even on relatively minor irregularities.